1. Field of the Invention
This invention relates to an electronic controlled fuel injection system suitable particularly for use in an internal combustion engine including a small number of cylinders.
2. Description of the Prior Art
Electronic controlled fuel injection systems have already been incorporated in spark ignition type gasoline engines. Such a fuel injection system comprises means for measuring the flow of air introduced to the engine, and a control circuit responsive to the intake air flow measurement for providing an injection pulse signal to each fuel injection valve in synchronism with rotation of the engine, the injection pulse signal having its pulse width determined in accordance with the intake air flow measurement so that the fuel injection valve can inject a suitable amount of fuel to the cylinder to provide an optimum airfuel ratio. In such a fuel injection system, normally, an inclined-plate type air flow meter has been used to measure the flow of air introduced to the engine. However, such a air flow meter is complex in structure and expensive to produce.
In view of demand for simple and durable flow meters, Karman's vortex flow meters have attracted special interest recently. Karman's vortex flow meters utilize the fact that the frequency at which Karman's vortexes are generated by a columnar object disposed in fluid flow is proportional to the speed of travel of the fluid. However, if disturbance appears in the fluid flow due to suction vacuum pulsation, vortexes would be irregularly produced which results in error introduced into the intake air flow measurement. When an engine is operating at or near full throttle, the pulsation damping effect of the throttle valve is reduced and the suction vacuum pulsation is transmitted through the throttle valve to the upstream portion of the intake passage so that large error is introduced into the intake air flow measurement. This is true particularly in internal combustion engines of the type having a small number of cylinders since they produce relatively large suction vacuum pulsation.